Recent vaccine studies in nonhuman primates have demonstrated the potential for virus-specific CD8 T cell responses to constrain or eradicate SIV infection. Rhesus CMV (RhCMV) and other persistently and non- persistently replicating vectors have contributed substantially toward these goals, but even the best current immunogens leave approximately 50% of animals unprotected. Realization of the full potential of T cell vaccines will require the development of novel immunogens that elicit T cell responses of sufficient magnitude, breadth, functionality and tissue distribution that virus replication can be effectively contained, preferably beginning in the initial hours and days of infection. We propose here a novel vaccine concept that builds upon the successes of recent preclinical vaccine trials. This new strategy is based on our laboratories' very recent and unexpected discovery that a novel set of evolutionarily conserved polypeptides is expressed from the 5' RNA leader sequence - previously termed the 5' untranslated region - of HIV-1 and the widely divergent SIV strains mac251/239, E660, SL92b and FTq, and that these peptides are highly immunogenic in vivo. HIV and SIV 5' leader-encoded polypeptides represent a new class of defective ribosomal products (DRiPs), which in other viral infections constitute an important source of peptides for antigen presentation and act as early sentinels of pathogen invasion. Because the 5' RNA leader sequence is part of every spliced and unspliced HIV-1 and SIV RNA message, it is the earliest and most abundant vRNA species in every productively infected cell, making peptides encoded by this region attractive as potential vaccine targets. The current grant proposal will explore this potential by testing the following hypothesis: Novel polypeptides encoded by 5' RNA leader sequences of HIV-1 and SIV are commonly expressed and are immunogenic in natural infection, and when expressed by a RhCMV vectored vaccine, will elicit T cell responses that confer protection in the Indian rhesus macaque - SIVmac239 infection model. Specific aims are: (i) To determine the prevalence, breadth and dynamics of T cell responses to 5' leader peptides in HIV-1 and SIV infection; (ii) To investigate the kinetics of expression and presentation of 5' leader peptides on infected cells and the efficiency of T cell recognition and virus control; (iii) To explore the conservation of 5' leader-encoded T cell epitopes and fitness costs associated with virus escape; and (iv) To evaluate the immunogenicity and protective efficacy of 5' leader-encoded polypeptides as vaccines. The implications of these studies extend beyond HIV/AIDS vaccine research: by demonstrating the expression and immunological recognition of a new class of 5' RNA leader-encoded DRiPs, we have uncovered a new paradigm of lentiviral gene expression relevant to studies of HIV-1 immunopathogenesis, latency and eradication.